1. Field of the Invention
The present invention relates generally to anionic photopolymerization, and to methods for anionic photoinitiation and anionic photoinitiators. More specifically, the present invention relates to substituted benzoylferrocene compounds useful as anionic photoinitiators for polymerizing or crosslinking monomers, oligomers, or polymers, to storage stable compositions containing these compounds, including photopolymerizable or photocrosslinkable compositions containing monomer, oligomer, or polymer and the photoinitiator, to methods for initiating anionic photopolymerization using these compounds and compositions, and to the methods for anionic photopolymerization themselves.
2. Description of Related Art
It is known that polymerization of monomers or oligomers, and the crosslinking of polymers, can proceed by several different mechanisms. These mechanisms can be loosely classified as radical polymerization and ionic polymerization. In radical polymerization, an initiating species is formed that is a free radical, i.e., that has an unpaired electron formed by the splitting of a chemical bond. As might be expected, the formation of a free radical requires the input of energy. However, once a sufficient number of initiating radicals are formed, they react with monomer, oligomer, or polymer species to generate other free radicals (i.e., they convert the monomer, oligomer, or polymer species into a free radical itself by breaking a chemical bond therein). The resulting free radical then reacts with other monomer, oligomer, or polymer molecules to form additional free radicals, while at the same time building or crosslinking the resulting polymer. The overall result is a chain reaction whereby, in the case of polymerization, a long chain molecule is built up from reactions between successive free radical species and additional monomer, oligomer, or polymer, terminating when two free radical species react, or when the reactive portion of the polymer encounters some other chain terminating group.
Ionic polymerization proceeds by the formation of cationic or anionic initiating species, which in turn react with monomer, oligomer, or polymer to generate other ionic species, again bringing about a chain reaction that results in the formation of long chain molecules that continue to grow until the reactive portions encounter and react with chain terminating groups. Ionic polymerization proceeds by removing or adding electrons from a molecule, generating net negative or positive charged species. Ionic polymerization proceeding by the formation of cationic reactive portions is called cationic polymerization, while ionic polymerization proceeding by the formation of anionic reactive portions is termed anionic polymerization.
In both radical and ionic polymerization, the initiating species may be formed from the monomer, oligomer, or polymer species itself, or may be formed from some other species, termed an initiator, that more readily forms the desired radical or ion. Initiators that yield active initiating species upon absorption of electromagnetic energy are termed photoinitiators, and the polymerization or crosslinking reactions that they initiate are termed photopolymerizations or photocrosslinking reactions. Mechanisms for both cationic and radical photopolymerization are suggested in S. Pappas, Encyclopedia of Polymer Science and Engineering, 2d ed. (Mark et al., eds.), 11:186-204 (1988).
Uncharged Lewis bases, such as ammonia or an organic amine also may initiate polymerization and crosslinking reactions. In this type of process, the base adds to the monomer, oligomer, or polymer to generate a zwitterion (a species containing positive and negative charges in different locations in the structure), which can initiate polymerization and crosslinking via a cationic, an anionic, or a hybrid (cationic and anionic) pathway.
Photopolymerization is the basis of important commercial processes that are widely applicable in a variety of different industries, including those using photoimaging technologies and coating and ink technologies. Photoinduced reactions of functionalized monomers, oligomers, and polymers play a prominent role in technologies that contribute an estimated $25 billion per year to the world economy. Important commercial applications include the ultraviolet curing of coatings and inks, the photoimaging of semiconductor chips and printed circuit boards, and the light driven storage and output of visual information. Examples of the latter include photopolymer-based printing plates (e.g., flexographic, letterpress, or relief printing plates, wherein the photopolymerization changes the solubility of the material, allowing an image to be formed by solvent washout), off-press color proofing (which simulates the images produced by a printing press, and wherein the photopolymerization changes either the solubility, tackiness, adhesion and cohesion, or electrical conductivity of the material, allowing images to be formed by washout, toning, delamination, or charging, toning, and transfer), and holographic recording (wherein images are formed when the photopolymerization changes the refractive index of the material). See Radiation Curing: Science and Technology, S. Pappas, ed., Plenum Press 1992, pp. 426-435.
Photoimaging involves the exposure of a photopolymerizable system to electromagnetic energy using a mask or pattern through which the electromagnetic energy must pass before it reaches the photopolymerizable system. A latent image of the mask is then imparted to the photopolymerizable system which can then be developed by dissolving or otherwise removing the unphotopolymerized material. The remaining photopolymerized layer forms a pattern of protected areas during subsequent chemical or physical manipulation of the underlying substrate. For example, photopolymerization is of crucial importance in the photolithography processes used to pattern semiconductors for etching and/or ion implantation, as well as for etching of, e.g., copper-containing printed circuit boards, etching of printing plates, etc. Photopolymerizable materials that are highly sensitive to the electromagnetic energy are used to pattern the material, in order to maintain acceptable throughput. The use of photopolymerization materials that are sensitive to relatively short wavelength electromagnetic radiation are desirable to obtain the sufficiently small patterning features needed in, e.g., semiconductor devices.
Photoinitiated, and in particular ultraviolet initiated, curing of coatings and printing inks can be based upon radical or cationic photopolymerization. Photoinitiated polymerization is commercially important for the curing of acrylate resins to provide coatings with controllable hardness, flexibility, and abrasion and solvent resistance. Ultraviolet curing provides rapid network formation and allows the use of heat-sensitive substrates (to be coated or printed upon), both of which are advantages over thermal curing. Ultraviolet curable inks can be used for lithographic, screen, flexographic, and letterpress printing. Ultraviolet curable coatings can be used as overprint varnishes, particle board finishes, metal decorative coatings, and to produce vinyl flooring. Photocurable coatings are useful in the preparation of dielectric coatings, protective coatings for electrical components, conductive coatings, and coatings for optical fibers.
Photopolymerization involves the use of electromagnetic energy (including light) to bring about polymerization of monomers, oligomers, and polymers, as well as the crosslinking of oligomers and polymers. Photochemical or photoinitiated reactions occur when a reactive species is produced on exposure of the reaction mixture to light or other electromagnetic radiation. The simplest mechanism for processes of this type involves the direct photochemical conversion of a reactant (monomer, oligomer, polymer, or mixture thereof) to a final product (Equation 1). If the reactant does not absorb the incident radiation, or does not form a reactive intermediate on exposure to the electromagnetic radiation, a second compound, referred to as a photoinitiator (P), can be added. The photoinitiator strongly absorbs the incident electromagnetic radiation and undergoes a photochemical transformation to form one or more reactive species I (Equation 2). Interaction of I with the reactant results in product formation (Equation 3). ##EQU1##
Since the photoinitiator and the reactant serve different functions, it is possible to optimize the properties of one without affecting the desirable features of the other. This inherent flexibility of a two-component system of the type shown in Equations 2 and 3 greatly simplifies the task of designing radiation-sensitive materials, and allows photopolymerization to be effectively used with a wider variety of polymerizable or crosslinkable reactants, such as those that lack a suitable chromophore for absorbing electromagnetic radiation, or that are inherently photoinert. The use of a photoinitiator that strongly absorbs incident radiation and that undergoes photochemical transformation to one or more reactive species I thus allows for a wider and more flexible use of photopolymerization.
The reactive species I can function as a true catalyst of the reaction, wherein it does not undergo any permanent change in structure or composition as a result of reaction, or it may be consumed while initiating the chain reaction of the reactant. It is also possible for species I to be present in sufficiently high localized concentrations to effectively function as a comonomer, although this is usually not the case. In any case, a reactive species I produced by the action of a single photon may result in the conversion of several reactant molecules to product, giving a quantum efficiency (i.e., the number of product forming events per photon absorbed) higher than 1. In effect, this results in the chemical amplification of the initial photochemical event, and affords a means of designing photopolymerizable materials with high radiation sensitivity.
The vast majority of commercially important photoinitiators are nonmetallic compounds that generate radicals and/or strong acids upon irradiation. These photoinitiators cause polymerization reactions to proceed via radical and/or cationic mechanisms. Well studied examples include benzoin and benzoin ethers, benzyl ketals, benzophenones plus hydrogen atom donors, thiol-ene systems, and onium salts belonging to the aryldiazonium, triarylsulfonium, and diaryliodium families. Of the relatively few transition metal-containing photoinitiators reported to date, most are organometallic complexes possessing photolabile ligands such as carbon onoxide, olefins, and carbocyclic rings. While the details of the mechanisms of initiation in these systems are not completely understood, the photoinduced formation of a coordinatively unsaturated metal center appears to be a central feature. See D. Yang and C. Kutal, Inorganic and Organometallic Photoinitiators in Radiation Curing: Science and Technology, S. Pappas, ed., Plenum Press 1992, pp. 21-55.
The ability of classical metal a(m)mine complexes to function as photoinitiators has been reported by Kutal, et al. In the Journal of Electrochemical Society, 34(9): 2280 (1987), Kutal and Willson reported that films spin-coated from solutions containing the copolymer of glycidyl methacrylate and ethyl acrylate along with the transition metal coordination complex [Co(NH.sub.3).sub.5 Br](ClO.sub.4).sub.2 undergo crosslinking upon irradiation at 254 nm and subsequent heating at 70.degree. C. The mechanism of crosslinking was determined to proceed in two distinct stages: (1) the primary photochemical process involving redox decomposition of the cobalt complex, and (2) one or more thermally activated reactions between the decomposition products and the pendant epoxide groups on the copolymer. The reactive species responsible for the photoinduced crosslinking by [Co(NH.sub.3).sub.5 Br](ClO.sub.4).sub.2 was not elucidated in this work, but it was hypothesized to be either a released ammonia molecule (neutral base catalysis) or cationic cobalt (II) complex (cationic catalysis).
In the Journal of Coatings Technology, July, 1990, Kutal, Weit, MacDonald, and Willson reported that Co(NH.sub.2 R).sub.5 X.sup.n+ complexes, where R is methyl or n-propyl and X is Cl.sup.- or Br.sup.-, photoinitiate crosslinking reactions in films of the copolymer of glycidyl methacrylate and ethyl acrylate at 254 nm. Irradiation of the cobalt complex at this wavelength causes efficient photoredox decomposition of the complex from a ligand-to-metal charge transfer excited state with release of several equivalents of free alkylamine. Even in the presence of oxygen, the decomposition quantum yields for the alkylamine cobalt complexes are uniformly higher than those reported for the comparable ammonia complexes. It was also observed that Co(NH.sub.2 Me).sub.5 X.sup.2+ exhibits a greater photosensitivity than Co(NH.sub.3).sub.5 X.sup.2+ in the crosslinking reaction, suggesting that the initiating species is the substituted amine or ammonia (neutral base catalysis), and that the sensitivity is a function of the basicity of the amine. See also Advances in Resist Technology and Processing VIII, Vol. 1446, pp. 362-367 (1991).
Storage stability has been problematic for some radical photoinitiators, particularly if all reactive species are not excluded from the photoinitiator composition. Moreover, cationic photoinitiators are only difficultly soluble in many monomer compositions. Radical polymerization is often inhibited, sometimes strongly, by the presence of oxygen (O.sub.2), making it's use under ambient conditions difficult or impossible. Anionic polymerization is typically less sensitive to this inhibition. Finally, some monomers and oligomers (e.g., .alpha.-cyanoacrylates, .alpha.-trifluoromethylacrylate) are insensitive to radical or cationic initiators, and can only be polymerized effectively by an anionic mechanism. These difficulties restrict the applicability and usefulness of radical and cationic polymerizations.
As a result of these and other considerations, photoinitiators that undergo photochemical release of anionic initiating species are of great value to inducing light-catalyzed polymerization or crosslinking of a wide range of monomers, oligomers, and polymers. For example, aldehydes and ketones, as well as certain ethylenically unsaturated monomers, undergo anionic polymerization or crosslinking, including ethylene, 1,3-dienes, styrene and .alpha.-methyl styrene, acrylates and methacrylates, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide. Certain monomers also undergo anionic ring-opening polymerization or crosslinking reactions, including N-carboxy-.alpha.-amino anhydrides, cyclic amides, cyclic esters, epoxides, and siloxanes.
Until relatively recently, such anionic photoinitiators were conspicuously absent from the catalog of available photoinitiators. Kutal, in U.S. Pat. No. 5,691,113, the entire contents of which is hereby incorporated by reference, disclosed inorganic transition metal complexes useful as anionic photoinitiators, including trans-[Cr(NH.sub.3).sub.2 (NCS).sub.4 ].sup.- (Reineckate anion) and trans-Cr(en).sub.2 (NCS).sub.2.sup.+ (where en is ethylenediamine). In U.S. Pat. No. 5,652,280, and in U.S. Ser. No. 08/900,815, the entire contents of each of which are hereby incorporated by reference, Kutal disclosed organic transition metal complexes useful as anionic photoinitiators, such as Pt(acac).sub.2 (where acac is acetylacetonate), optionally substituted ferrocenes, and optionally substituted ruthenocenes.
Japanese publication 09-249708 discloses photocurable resin formulations containing metallocenes having a Group VIII transition metal and aromatic ligands such as .pi.-arenyl, indenyl, or optionally substituted .eta.-cyclopentadienyl groups. The curing reaction is disclosed to proceed by an anionic polymerization.
Despite the existence of these disclosures of various anionic photoinitiators, there remains a practical need in the art and in the industries that use photopolymerization for anionic photoinitiators having improved storage stability and shelf-life. In particular, there remains a need for anionic photoinitiators having both enhanced thermal stability and good photosensitivity.
It is accordingly an object of the present invention to provide novel compounds suitable as anionic photoinitiators, but having thermal and storage stability that is improved over currently known anionic photoinitiators, while maintaining good photosensitivity.
It is another object of the present invention to provide compositions containing these anionic photoinitiators, including photopolymerizable compositions that possess the increased storage stability described above.
It is yet another object of the present invention to provide methods of initiating photopolymerization using the compounds and compositions of the present invention.
It is yet a further object of the present invention to provide methods of photopolymerizing monomers and/or oligomers, and/or of photocrosslinking oligomers and/or polymers, using the anionic photoinitiating compounds and compositions of the present invention.